The present Applicant has developed a plethora of thermal bubble-forming printheads and thermal bend-actuated printheads. The Applicant's thermal bubble-forming printheads include those with suspended heater elements (as described in, for example, U.S. Pat. No. 6,755,509; U.S. Pat. No. 7,246,886; U.S. Pat. No. 7,401,910; and U.S. Pat. No. 7,658,977, the contents of which are incorporated herein by reference) and those with embedded or bonded heater elements (as described in, for example, U.S. Pat. No. 7,377,623; U.S. Pat. No. 7,431,431; US 2006/250453; and U.S. Pat. No. 7,491,911, the contents of which are incorporated herein by reference). The Applicant's thermal bend-actuated printheads typically have movable paddles defined in a nozzle plate of the printhead (as described in, for example, U.S. Pat. No. 7,926,915; U.S. Pat. No. 7,669,967; and US 2011/0050806, the contents of which are incorporated herein by reference).
The Applicant's Memjet® printheads are characterized by a relatively high nozzle density compared to commercially-available printheads. Memjet® printheads typically comprise a plurality of color planes with a pair of offset nozzle rows in each color plane. A key advantage of Memjet® printheads is the relatively narrow print zone resulting from integration of a plurality of color planes on each printhead integrated circuit. The color planes of the Memjet® printhead are spaced closely together, which obviates any alignment problems between the color planes during dot-on-dot printing. By contrast, widely spaced color planes necessitate complex media feed mechanisms to achieve proper alignment, adding to the complexity of other known pagewidth printing systems (e.g. HP Edgeline).
Typically, a distance between nozzles rows from neighboring color planes in a Memjet® printhead is in the range of 25 to 200 microns or 50 to 100 microns. This close spacing of color planes produces problems which are unique to such printheads. A significant problem of closely spaced nozzle rows from different color planes is one of color mixing on the nozzle plate of the printheads. If, for example, black ink mixes into a yellow ink on the nozzle plate, then this may adversely affect print quality as well as impacting on printhead maintenance routines. The present Applicant has identified two mechanisms which manifest in adverse ink mixing on the nozzle plate: ink wetting onto the nozzle plate; and fibres or particulates bridging between rows of nozzles.
Hitherto, the problem of ink mixing on the nozzle plate of inkjet printheads has been addressed in the art by applying hydrophobic surface treatments to the nozzle plate. For example, the present Applicant has described nozzle plates having a coating of a hydrophobic siloxane polymer, which is applied during MEMS fabrication of the printhead (see, for example, U.S. Pat. No. 7,938,974; U.S. Pat. No. 7,669,967, the contents of which are herein incorporated by reference). However, such coatings add to the cost and complexity of printhead fabrication. Moreover, they do not control a direction of color mixing so as to favor acceptable color mixing over adverse color mixing.
It would be desirable to provide a means for minimizing adverse color mixing on inkjet nozzle plates, which does not rely on nozzle plate coating treatments. Alternatively, it would desirable to provide a means for minimizing adverse color mixing on inkjet nozzle plates, which complements and/or improves the effectiveness of existing nozzle plate coating treatments.